1. Field of the Invention
The present invention relates to a method of utilizing a methane-containing biogas, which may be selected from the group consisting of waste dump gas and biogas originating from fermentation installations and putrefication processes of sewage treatment plants, by feeding the methane-containing biogas to a gas engine of a gas engine/generator assembly generating electricity.
2. Description of the Prior Art
It is known to install gas engine/generator assemblies in waste dumps and to operate the gas engine with waste dump gas. The waste dump gas sucked out of the waste has a content of about 50%, by volume, of methane. The remainder is comprised substantially of CO2 and some nitrogen. When the waste dump is closed, the production of biogas from the waste is slowly reduced over a period of about 10 years. The installed gas catching system then increasingly sucks air into the waste so that the methane content is reduced. When the methane content is reduced below 40%, by volume, it is impossible to operate the gas engine with this lean gas, and the waste dump gas must be burned off.
As the biogas is sucked off from its generating source, CO2 is removed from the process generating the biogas, the CO2 being an educt for the bioreactions proceeding under anaerobic conditions in the presence of organic materials and microorganisms. Therefore, the removal of CO2 has several disadvantages. In the first place, CO2 is an undesirable inert gas component when the methane fuel is burned in the gas engine. In the second place, the removed CO2 is no longer available as an educt for the bioreaction proceeding in the waste dump or in the fermentation installations and putrefication processes of sewage treatment plants. Furthermore, in the operation of a waste dump the removal of CO2 favors the penetration of air into the waste and thus produces an undesired dilution effect.
It is the primary object of this invention to utilize methane-containing biogases in an economical manner for the generation of electricity. In particular, the method enables a biogas whose methane content is below 40%, by volume, to be used for generating electricity.
In a method of the first-described type, this and other objects are accomplished according to the invention by the steps of passing the biogas through a membrane separating installation to divide the biogas into a first gas stream having a higher methane content than the biogas fed thereto and a second gas stream enriched in CO2, feeding the first gas stream to the gas engine as fuel, and returning the second gas stream to the source of the biogas.
Membranes for separating gases on the basis of very different permeabilities for methane and carbon dioxide are commercially available, and their use in the membrane separating installation enables the effective concentration of the methane content in one of the gas streams in the installation. The biogas is condensed and is fed to the gas permeation module of a membrane separation installation whose membranes have a preferred permeability for CO2. The residue retained on the pressure side of the gas permeation module is fed to the gas engine while the permeate in returned to the biogas source.
This method very economically utilizes a biogas which is a lean gas containing less than 40%, by volume, of methane. It enables the gas fueling the gas engine to be sufficiently enriched in methane that it may be operated under optimal conditions, for example at an air ratio of xcex=1.5 to 1.8, preferable 1.6. Conventional gas engines operate at such an air ratio with high efficiency and low exhaust gas emissions.
The method of this invention may be used advantageously in connection with waste dumps as the source of the biogas, particularly closed waste dumps. The return of the gas stream enriched with CO2 to the waste prevents to a large extent the penetration of air into the waste, which is caused by the progressively decreasing production of methane. This has the advantage that the anaerobic process proceeding in the waste is not disturbed by air. In this manner, a very effective methane production may be maintained even in aging or closed waste dumps.
It is another advantage of the method of the invention that the heat and humidity required for the bioreaction and production of methane may be optimized with the aid of the gas stream returned to the waste. To obtain this result according to a preferred feature of the present invention, the second gas stream returned to the waste is heated by the waste heat produced by the gas engine, for example, and/or is humidified with water, which may be derived from the waste dump itself.
The return of the CO2-enriched gas stream to the bioreaction in the waste increases the CO2 partial pressure in the process of generating the methane, which enhances the methane production because the microorganisms in the waste produce methane with CO2 as educt and the hydrogen-containing organic materials. In this way, the method has a considerable advantage in optimizing the operation of putrefication towers in sewage treatment plants. In addition to fueling gas engines economically with biogas, the methane production is enhanced, which shortens the dwell time of the waste in the putrefication tower and thus increases its capacity.